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A federation engine for virtually integrating SPARQL endpoints

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/*******************************************************************************
 * Copyright (c) 2019 Eclipse RDF4J contributors.
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Distribution License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 *******************************************************************************/
package org.eclipse.rdf4j.federated.optimizer;

import java.util.ArrayList;
import java.util.List;

import org.eclipse.rdf4j.federated.algebra.EmptyNUnion;
import org.eclipse.rdf4j.federated.algebra.EmptyResult;
import org.eclipse.rdf4j.federated.algebra.NUnion;
import org.eclipse.rdf4j.federated.exception.OptimizationException;
import org.eclipse.rdf4j.federated.structures.QueryInfo;
import org.eclipse.rdf4j.query.algebra.TupleExpr;
import org.eclipse.rdf4j.query.algebra.Union;
import org.eclipse.rdf4j.query.algebra.helpers.AbstractSimpleQueryModelVisitor;

/**
 * Optimizer to flatten the UNION operations.
 *
 * @author Andreas Schwarte
 *
 */
public class UnionOptimizer extends AbstractSimpleQueryModelVisitor implements FedXOptimizer {

	protected final QueryInfo queryInfo;

	public UnionOptimizer(QueryInfo queryInfo) {
		super(true);
		this.queryInfo = queryInfo;
	}

	@Override
	public void optimize(TupleExpr tupleExpr) {
		tupleExpr.visit(this);
	}

	@Override
	public void meet(Union union) {

		// retrieve the union arguments, also those of nested unions
		List args = new ArrayList<>();
		handleUnionArgs(union, args);

		// remove any tuple expressions that do not produce any result
		List filtered = new ArrayList<>(args.size());
		for (TupleExpr arg : args) {
			if (arg instanceof EmptyResult) {
				continue;
			}
			filtered.add(arg);
		}

		// create a NUnion having the arguments in one layer
		// however, check if we only have zero or one argument first
		if (filtered.isEmpty()) {
			union.replaceWith(new EmptyNUnion(args, queryInfo));
		} else if (filtered.size() == 1) {
			union.replaceWith(filtered.get(0));
		} else {
			union.replaceWith(new NUnion(filtered, queryInfo));
		}
	}

	/**
	 * Add the union arguments to the args list, includes a recursion step for nested unions.
	 *
	 * @param union
	 * @param args
	 */
	protected void handleUnionArgs(Union union, List args) {

		if (union.getLeftArg() instanceof Union) {
			handleUnionArgs((Union) union.getLeftArg(), args);
		} else {
			args.add(union.getLeftArg());
		}

		if (union.getRightArg() instanceof Union) {
			handleUnionArgs((Union) union.getRightArg(), args);
		} else {
			args.add(union.getRightArg());
		}
	}

}